This disclosure describes a method and an apparatus for improving light exposure, e.g., ultraviolet exposure of photo-curable printing plates, e.g., photopolymer flexographic printing plates, letterpress plates and other polymer printing plates, as well as polymer sleeves and polymer coated printing cylinders. Photo-curable, of course, means curable by photons, e.g., light, e.g., light in the ultraviolet range or some other range.
Photopolymer plates have found a broad range of applications. A variety of different methods can be applied for transferring an image for printing, e.g., in the form of imaging data, to a polymer plate. For example, an image mask, which can be a film applied to the surface of the plate while the plate is exposed, or a layer directly on top of the polymer surface is laser ablated layer directly on top of the polymer surface, is placed on top of a polymer sheet.
By a digital plate is meant a plate that is exposed to imaging data by ablating a mask material that is on the plate, e.g., by exposure to laser radiation in an imaging device. The process of producing a digital plate is called a digital process herein.
By a conventional analog plate is meant a plate that is exposed to imaging data by exposing photographic film according to the imaging data, and then using the film to form a mask during exposure to curing radiation. The process of producing a conventional analog plate is called an analog process herein.
Irrespective of the way imaging data is transferred to the plate, the plate needs light, e.g., UV light for curing. Such UV curing is currently done by one of several different methods. After curing, the non-cured portions of the polymer are removed, either using solvents, or by melting the non cured material through heat treatment and absorption with a web.
Polymer printing plates are three dimensional, that is, include a depth dimension from the printing surface. Small printing details on the plate's surface carry ink for printing. Analog plates, i.e., plates produced using a conventional analog process can produce small features that have printing features that are substantially flat. Such a feature is called a flat top herein. It is much more difficult to produce such flat tops on digital plates, i.e., using a digital process. Features on digital plates tend to have rounded surfaces that extend down in depth. Such a feature is called a round top herein.
It would be advantageous to have a method and apparatus that allows the shape of the three dimensional printing features, such as halftone dots and other structures on the printing plate to be controlled, e.g., so that round tops or flat tops can be produced on a digital plate as an operator choice.
Commonly owned U.S. Application Publication No. US 2009-0294696 A1 published Dec. 3, 2009 and titled CURING OF PHOTO-CURABLE PRINTING PLATES WITH FLAT TOPS OR ROUND TOPS describes an apparatus and methods that take advantage of the observation made by the inventor that one way to control the shape of haltone dots is to have a method and apparatus that provides for illuminating an exposed photopolymer plate at two levels, a first high intensity followed by illumination by a lower intensity. Having UV sources, arrangements, and control systems described in US 2009-0294696 A1 may be expensive.